Physiological and molecular evidence that environmental changes elicit morphological interconversion in the model diatom Phaeodactylum tricornutum.

Over the last decades Phaeodactylum tricornutum has become a model to study diatom biology at the molecular level. Cells have the peculiarity to be pleiomorphic and it is thought that this character is triggered by culture conditions, although few quantitative studies have been performed and nothing is known at the molecular level. Our aim was to quantify the effect of growth conditions on cell morphology of different P. tricornutum strains by quantitative microscopy, cellular imaging, and non-targeted transcriptomics. We show that morphotype changes can be regulated by changing culture conditions, depending on the strain, and show a common trend of increased oval cell abundance as a response to stress. Examination of expressed sequence tags (ESTs) from triradiate cells infers the importance of osmoregulation in the maintenance of this morphotype, whereas ESTs derived from oval cells grown in hyposaline and low temperature conditions show a predominance of genes encoding typical components of stress pathways, especially in signaling, cell homeostasis and lipid metabolism. This work contributes to better understand the importance of the unique capability of morphotype conversion in P. tricornutum and its relevance in acclimation to changing environmental conditions.

Digital expression profiling of novel diatom transcripts provides insight into their biological functions.

BACKGROUND: Diatoms represent the predominant group of eukaryotic phytoplankton in the oceans and are responsible for around 20% of global photosynthesis. Two whole genome sequences are now available. Notwithstanding, our knowledge of diatom biology remains limited because only around half of their genes can be ascribed a function based onhomology-based methods. High throughput tools are needed, therefore, to associate functions with diatom-specific genes. RESULTS: We have performed a systematic analysis of 130,000 ESTs derived from Phaeodactylum tricornutum cells grown in 16 different conditions. These include different sources of nitrogen, different concentrations of carbon dioxide, silicate and iron, and abiotic stresses such as low temperature and low salinity. Based on unbiased statistical methods, we have catalogued transcripts with similar expression profiles and identified transcripts differentially expressed in response to specific treatments. Functional annotation of these transcripts provides insights into expression patterns of genes involved in various metabolic and regulatory pathways and into the roles of novel genes with unknown functions. Specific growth conditions could be associated with enhanced gene diversity, known gene product functions, and over-representation of novel transcripts. Comparative analysis of data from the other sequenced diatom, Thalassiosira pseudonana, helped identify several unique diatom genes that are specifically regulated under particular conditions, thus facilitating studies of gene function, genome annotation and the molecular basis of species diversity. CONCLUSIONS: The digital gene expression database represents a new resource for identifying candidate diatom-specific genes involved in processes of major ecological relevance.